Switched capacitor (SC) circuits are used to implement several signal-processing functions, ranging from filters to analog-to-digital converters. The input stage of a SC circuit can be modelled as a capacitor CL that must be charged at every clock cycle. Since the equivalent input impedance of a SC circuit may be low (due to large capacitance CL and/or high clock frequency), a driver is needed to isolate the signal source from the SC circuit. The driver is a continuous-time circuit that charges the input capacitor CL to a voltage level proportional to the input signal. Since the information in a SC circuit is represented by the charge on the internal capacitor, the driver must ensure that the voltage on the input capacitance CL has properly settled at the end of the charging phase.
A typical SC driver uses an amplifier in feedback configuration to charge the capacitor CL during the charging phase. If the output impedance of the amplifier and the resistance of the switch that is driven by the charge phase control signal are low enough, the charging transient is fast and the input voltage will be copied accurately on the capacitor CL. However, if the output impedance of the amplifier (or the aforementioned switch resistance) is not low enough, a charging transient occurs. In this case, if the bandwidth of amplifier and feedback network are wide, the voltage on the capacitor settles to the proper value at the end of the transient.
Implementing an amplifier with low output impedance entails significant costs. Reducing the output impedance of the amplifier requires increasing the power and the dimension of the output devices of the amplifier. To counteract the effect of the high output impedance, a wide-bandwidth amplifier may be used. However, this would also lead to an increase in the power consumption of the amplifier. Moreover, if there are stringent requirements on the input-referred offset of the driver, the amplifier must have a low intrinsic offset or be offset-compensated. In the first case, the amplifier will require a large area to obtain a good matching of its input device and, consequently, a low input offset. In the latter case, it must be implemented using offset-compensation techniques, such as chopping or auto-zeroing. However, it is well-known that such techniques limit the bandwidth of amplifiers (cf. Enz, C. C., Temes, G. C., “Circuit techniques for reducing the effects of op-amp imperfections: autozeroing, correlated double sampling, and chopper stabilization,” Proceedings of the IEEE, vol. 84, no. 11, pp. 1584, 1614, November 1996). This can be avoided only at the expense of increased circuit complexity (cf. Qinwen Fan; Huijsing, J. H.; Makinwa, K. A A, “A 21 nV/√Hz Chopper-Stabilized Multi-Path Current-Feedback Instrumentation Amplifier With 2 μV Offset,” Solid-State Circuits, IEEE Journal of, vol. 47, no. 2, pp. 464, 475, February 2012). Accordingly, there may be a need for a driver for switched capacitor circuits, which is capable of driving the switched capacitor circuit properly without the need for an amplifier with large bandwidth and/or low output impedance.